This invention is directed to a breathable multilayer film having a core layer and one or two adjacent skin layers. The core layer contains a mixture of thermoplastic polymer and particulate filler, and is rendered breathable to moisture vapor when the film is stretch-thinned following coextrusion. The skin layer or layers prevent filler particles from the core layer from building up at the die lip during coextrusion. The skin layers crack and/or break during stretching to increase the breathability of the overall film.
Breathable stretch-thinned films, and laminates including a breathable film and a nonwoven web, are used in diaper backings, other personal care products, and medical garments. U.S. Pat. No. 5,695,868, issued to McCormack, discloses a breathable film and a breathable, cloth-like film/nonwoven composite which includes the film thermally bonded to a fibrous polyolefin nonwoven web. Typically, the film includes a core layer coextruded with one or two adjacent skin layers.
The core layer, which accounts for most of the film thickness, can be formed from a mixture of a thermoplastic polymer with a substantial quantity (e.g., 30-75% by weight) of a particulate inorganic filler. When the film is stretched subsequent to coextrusion, voids form around the filler particles in the core layer. The voids are somewhat defined and separated by thin polymer membranes which permit molecular diffusion of water vapor through the film. This diffusion is what causes the film to have water vapor breathability.
The skin layer or layers account for a minor percentage of the film thickness, and may constitute less than 15% of the film thickness. The skin layers serve two main purposes. One purpose is to partially shield the die lips from the core layer during coextrusion, so that filler particles from the core layer do not excessively build up at the die lips. Another purpose is to serve as a bonding layer, which facilitates bonding of the coextruded film to a nonwoven web.
One disadvantage of skin layers is that they reduce the moisture vapor breathability of the overall film. The thicker the skin layers, the greater the reduction in breathability. Optimization of skin layer thickness requires providing enough skin layer thickness to control die build-up during extrusion and provide bonding to a nonwoven web, yet not so much skin layer thickness that overall breathability is substantially impaired. This can be a difficult balance to achieve, especially in view of the trend toward more breathable films and laminates.
The present invention is directed to a breathable, stretch-thinned film including a core layer and one or two adjacent skin layers. The core layer is made from a mixture of one or more thermoplastic polymers and a particulate filler, such that voids are formed around the filler particles when the film is stretched. The skin layers are formed from a polymer blend which maintains its integrity during coextrusion of film, yet which cracks when the film is stretched.
As a result of this invention, the skin layers can be made thick enough to adequately prevent extrusion die build-up caused by the particulate filler in the core layer. The skin layers can also be as thick as necessary to provide adequate bonding of the film to a substrate, such as a nonwoven web. Because the skin layers crack during stretching of the film, their presence does not significantly impair the moisture vapor breathability of the overall film.
The skin layer(s) are made from a blend of two or more incompatible polymers which cause cracking of the skin layer(s) when the film is stretched. In one embodiment, the skin layer(s) are made from a blend containing about 20-80% by weight branched low density polyethylene made using a conventional high-pressure polymerization process, and about 20-80% by weight of an ethylene-propylene copolymer elastomer which contains a major percentage of propylene and a minor percentage of ethylene. In another embodiment, the skin layer(s) are made from a blend containing about 20-80% by weight of an ethylene vinyl acetate copolymer containing up to about 12% by weight vinyl acetate, and about 20-80% by weight of an ethylene-propylene copolymer elastomer as described above. In another embodiment, the skin layer(s) are made from a blend containing about 20-80% by weight of an ethylene methyl acrylate copolymer, and about 20-80% by weight of an ethylene-propylene copolymer elastomer as described above.
In still further embodiments, the skin layer(s) are made from a blend containing about 75-98% by weight of a first component selected from branched low density polyethylene, ethylene vinyl acetate containing up to about 12% by weight vinyl acetate, and ethylene methyl acrylate containing up to about 12% by weight methyl acrylate; and about 2-25% by weight of a second component selected from polypropylene, polystyrene and polybutene.
The present invention also includes a method of preparing a stretch-thinned breathable film. The method includes the steps of providing a core layer composition containing one or more thermoplastic polymers and a particulate filler, providing a skin layer composition for one or both sides of the core layer, coextruding the core layer composition and skin layer composition together through a die to form a multilayer film having a core layer and at least one skin layer, stretching the film in at least one direction after it leaves the die, and cracking the skin layer during stretching of the film. An important feature of the method is that the skin layer is cracked during stretching, after it leaves the die, instead of being cracked at the die during extrusion.
With the foregoing in mind, it is a feature and advantage of the invention to provide a breathable stretch-thinned film having a core layer and a cracked skin layer on one or both sides of the core layer, the skin layer(s) being cracked during stretching of the film to provide enhanced breathability to moisture vapor.
It is also a feature and advantage of the invention to provide a breathable stretch-thinned film having improved processability, whose skin layers are thick enough to substantially prevent die build-up of particulate filler from the core layer.
It is also a feature and advantage of the invention to provide an improved method of making a multilayer stretch-thinned breathable film, in which the skin layer(s) are cracked during stretching of the film to provide enhanced breathability.
The foregoing and other features and advantages will become further apparent from the following detailed description of the presently preferred embodiments, read in conjunction with the drawings.